This invention relates generally to digital logic circuits using superconducting circuit elements and, more particularly, to digital logic circuits using nonhysteretic superconducting circuit components. Superconducting circuit elements known as Josephson junctions have been used, or proposed for use, in a wide variety of high-speed computational applications, including digital logic, random access memories, and analog-to-digital converters. Josephson junctions employing various metals that have superconductive properties at very low temperatures exhibit a characteristic hysteresis effect, which has been used to advantage in the design of high-speed digital logic.
Hysteretic devices include a common three-layer structure comprising superconductor, oxide and superconductor. Nonhysteretic devices were known, but not widely used, and include point contact devices, weak-link microbridges, high current density oxide barrier junctions, and three-layer structures with semiconductor barrier layers such as Si or Ge-Sn. Although hysteresis can complicate circuit design, it does provide a device with two well-defined logic states. In addition, the hysteresis gives high effective current gain in the switching elements; that is to say a small control signal can result in a large output signal. In contrast, the same circuits made from nonhysteretic junctions have greatly reduced gain and ill-defined logic levels.
In recent years, researchers have discovered that various ceramic materials exhibit superconductivity at much higher temperatures than metals, allowing the operation of superconducting circuits with lower cooling requirements and higher overall energy efficiencies. However, these newer superconducting materials are of the nonhysteretic type, and previously developed logic designs for superconducting devices are no longer applicable.
By way of further background, a basic superconducting circuit element frequently used in digital logic is known by the acronym SQUID, for superconducting quantum interference device. SQUIDs depicted in this specification are of the direct current (dc) type, having two or more Josephson junctions and at least one control inductor. Depending on the magnitude of a control current passed through the inductor and the magnitude of a bias current passed through the junctions, the SQUID can be made to operate in either a superconducting state, with a practically zero impedance, or a conventional resistive state.
One technique that has been suggested for improving the performance of nonhysteretic superconductive circuits is described in U.S. Pat. No. 4,342,924 to Howard et al. A positive feedback path is used to enhance the effect of a control current used to switch from the superconductive to the resistive state. Although the use of positive feedback in this way enhances operation of a nonhysteretic superconducting circuit, the Howard et al. patent offers no general solution to the problem of circuit design with nonhysteretic devices.
It will be appreciated from the foregoing that there is a need for a new technique employing nonhysteretic superconducting elements in digital logic and related circuitry. The present invention is directed to this end.